From what I learned from Bronwen last weekend, make a simple syrup as a base and add the rum (or other alcohol of choice) to the syrup (cooled a bit) and blend, then gently introduce into the cooled cake. For the basic simple syrup, check any bartending site.

If you add just straight rum it will not really permeate the cake (being sooooo loose), it will just pool down in it and make the bottom soggy (and may even start to collapse).

I once tried using straight rum, but did not like it. If you want it really wet, it will be too strong and you will need a lot, hence expensive. I decided (for a second attempt) to use a simple syrup (1 cup water + 1/2 cup sugar), heat until sugar dissolves, let cool down and then add about 1/4 cup rum. This was better and I had enough liquid to soak the cake through. Try a dark or spiced rum for a nicer taste than white rum.

Mix 1 cup of sugar, 2 Cups of Water. Bring to a Boil. Make sure Sugar is dissolve, Let it cool, Then add a Tablespoon of the Liquor you desire to put( White Rum is Good. For a Wedding Cake you can use Brandy & A Teaspoon of Vanilla Or Almond Extract, Once Everything is mix, With a Brush, Brush the Syrup on the Cake. I hope This Helps.

Simple syrup is equal parts water and sugar by weight, brought just to a boil so that the sugar dissolves. Since you're planning on adding rum to it, that would dilute it some, but that would probably help it soak in the cake a little better.